Because of having high sensitivity, capacitive sensing devices have been widely used in electronic devices, such as Personal Digital Assistants (PDAs), mobile phones, tablets, computers, or the like. For example, a mobile phone may use a capacitive sensing touch screen as an input device, and may also use a capacitive proximity sensor for sensing proximity of an object, such as a human finger, a human face or the like.
Capacitance of a capacitive sensing device may vary with approaching of an object, e.g., a human finger, and therefore the device may sense proximity of the object. However, such high sensitivity may cause problems. For example, a capacitive sensing device may be susceptible to environmental changes because the capacitance may be easily changed due to the environmental changes, such as humidity, temperature, or dirt changes. As a result, if the environmental changes cause a change of the capacitance towards the same direction as that caused by the approaching object, i.e., both the capacitance values due to the approaching object and the environmental changes are positive or negative, the capacitive sensing device may become too sensitive. Furthermore, if the environmental changes cause a change of the capacitance towards an opposite direction compared with that caused by the approaching object, i.e., one of the capacitance values due to the approaching object and the environmental changes is positive and the other is negative, the capacitive sensing device may become too insensitive.
It is undesirable that environmental changes induce a capacitance change that may be interpreted as proximity of an object when actually such an object does not exist. Conversely, it is also undesirable that environmental changes cause a capacitance change due to an approaching object to be much smaller so that the proximity of the object may not be detected. Thus, there is a need for reducing the influence of environmental changes on the stability and sensitivity of a capacitive sensing device.